1. Field of the Invention
This invention involves solidification of molten materials to yield desirable physical and/or compositional configurations.
2. Disclosures of Interest
Compositional changes which occur upon solidification of molten materials comprising two or more components are well known and have been studied in detail. (See, for example, Bruce Chalmers, Principles of Solidification, John Wiley and Sons, Inc., New York, 1964, especially pp. 120-180 and references cited therein.) If one of the components of the melt has a higher solubility in the liquid phase than in the solid phase, that component will be "rejected" as a freezing liquid-solid interface advances through the melt during solidification. (See, for example, Principles of Solidification, ibid., pp. 128-143.)
On Mar. 20, 1956, U.S. Pat. No. 2,739,088 was granted to W. G. Pfann for a process of producing compositional and/or physical changes in a body by causing a molten region to travel through the body. In an embodiment of that process, a portion of an appropriate body of material such as, for example, a cylindrical body of semiconductor material, is rendered molten by a heat source which may be made to traverse the body. Traversal of the heat source results in concomitant motion of the molten region through the material, melting solid material in its path and leaving refrozen solid material in its wake. The molten material generally comprises at least two components, and depending upon the phase equilibrium properties associated with this two component material, the refrozen material left in the wake of the molten region may be of higher purity than before the process, or may be doped with desirable impurities as a result of the passage of the molten zone.
On Nov. 12, 1957, W. G. Pfann was further granted U.S. Pat. No. 2,813,048 to a zone melting process descriptively titled "Temperature Gradient Zone Melting" (TGZM). In this process, a traversing heat source is not necessarily used to move the molten zone through the body, but rather an appropriate temperature gradient, which may remain stationary, results in traversal of the molten zone through the body. In the TGZM process, the temperature gradient which is impressed upon the body, in conjunction with phase equilibrium properties associated with the multi-component material, results in a molten region with a spatial variation in concentration of at least one of the components, for example, the solute. At the hotter of the liquid-solid interfaces associated with the molten region equilibrium solute concentrations are lower than at the cooler of the liquid-solid interfaces associated with the molten region. The resultant diffusion of the solute from the cooler liquid-solid interface through the molten region to the hotter liquid-solid interface, in an attempt to reach an approximately uniform concentration, tends to raise the solute concentration at the hotter liquid-solid interface above that value determined by the liquidus for the composition. Solvent material is then dissolved at the hotter liquid-solid interface in an attempt to return the liquid to the liquidus concentration. Similarly, diffusion of the solute away from the cooler interface lowers the solute concentration at that interface and solvent material is refrozen at the cooler interface in an attempt to return that interface to the liquidus concentration. In this manner, the molten region advances through the body melting solid material at the hotter interface and refreezing liquid material at the cooler interface.
On Mar. 24, 1981, K. A. Jackson, L. C. Kimerling, and H. J. Leamy were granted U.S. Pat. No. 4,257,824 to an improved temperature gradient zone melting process. In this process, the molten zone is preferentially heated thereby selectively establishing a temperature gradient across the molten zone. Such selective establishment of a temperature gradient across the molten zone may be effected by using a laser as the heat source. In specific embodiments, the molten zone is essentially the only region of the material which absorbs energy from the laser light beam. In any event, as in previous temperature gradient zone melting processes, a molten region is passed through the body leaving in its wake a solid region of alloy whose composition is determined by the characteristics of both the heating mechanism and the phase diagram association with the material.
The rejection of solute at the liquid-solid interface of an advancing solidification front may result in regions of high solute concentration at grain boundaries. This phenomenon, known as grain boundary segregation, results from segregation of the solute in the last volume to solidify, and is exemplary of a number of different segregation phenomena associated with the advance of a liquid-solid interface upon resolidification. (See, for example, Principles of Solidification, ibid., pp. 171ff.) Most of these phenomena, however, are associated with random events which cannot be easily controlled. Hence, enriched solute regions of a predetermined physical configuration were apparently not amenable to easy formation using the characteristic phenomena which occur during redistribution of a solute upon solidification.